Conventional dielectric porcelain materials include ZrO.sub.2 -SnO.sub.2 -TiO.sub.2 and BaO-TiO.sub.2 systems, as well as such systems in which part of the constituent elements are replaced by other elements, and the combination of a dielectric porcelain or glass having a positive temperature coefficient for specific inductivity with TiO.sub.2 which has a negative temperature coefficient so as to control the temperature coefficient of the first component. However, these conventional dielectric porcelain materials have relatively high specific inductivities in the range of from 30 to 40.
As the operating range of dielectric oscillators has expanded to that of the microwave frequencies, there is an increasing demand for porcelain materials that have lower specific inductivities and which hence are easier to work.
Alumina porcelain holds much promise for use as a low specific inductivity material. One problem with alumina porcelain is that in spite of its relatively low specific inductivity (.congruent.10), it has an extremely high temperature coefficient (-60 ppm/.degree.C.) for resonating frequency.
The present inventors previously filed Japanese Patent Application No. 151589/84 wherein they described an alumina porcelain composition of the Al.sub.2 O.sub.3 -CaTiO.sub.3 -SrTiO.sub.3 system that has excellent properties as a high frequency dielectric material. This composition has the compositional range defined by points A, B, C and D in the three-component composition diagram accompanying the specification of that application, wherein the points A, B, C and D represent the following molar fractions of Al.sub.2 O.sub.3, CaTiO.sub.3 and SrTiO.sub.3 :
______________________________________ Al.sub.2 O.sub.3 CaTiO.sub.3 SrTiO.sub.3 ______________________________________ Point A 0.99 0.01 0 Point B 0.96 0.04 0 Point C 0.91 0.04 0.05 Point D 0.91 0.01 0.08 ______________________________________
This alumina porcelain composition consists of a binary system of an alumina sinter and calcium titanate (CaTiO.sub.3) or a ternary system of strontium titanate as combined with the binary system. The alumina sinter has a negative temperature coefficient for resonating frequency (said coefficient is hereunder abbreviated as .tau..sub.f) whose absolute value is as great as 60 ppm/.degree.C. while exhibiting a high unloaded Q, a small dielectric loss at high frequencies, as well as high heat conductivity, mechanical strength and chemical stability. The calcium titanate which is present in a small amount is formed from CaO and TiO.sub.2 and has a positive temperature coefficient (.tau..sub.f) close to +900.